Catalytic cracking is used in most U.S. refineries to increase gasoline yield from petroleum feedstocks. The catalytic cracker accepts diverse feedstocks, including heavy and light vacuum gas oil (VGO) from the vacuum distillation column, heavy coker gas oils, light gas oil from the atmospheric distillation column, solvent deasphalted oils, lube extracts and hydrocracker bottoms. The catalytic cracking operation breaks down the heavy molecules in these cuts to form lower boiling components for gasoline and heating and fuel oils. The cracking operation can be adjusted to meet fluctuating consumer demand for these products.
The dominant catalytic cracking process is fluid catalytic cracking (FCC), so called because the catalyst is in the form of fine grains that circulate through the reactor continuously in fluid-like flow. In the FCC process, preheated hydrocarbon feedstock is injected into a section of the reactor unit known as the riser. Hot catalyst flows upward through the riser, and the cracking reactions take place. Numerous products can be retrieved from the raw crackate. These range from hydrogen to heavy fuel oils.
The raw effluent from the reactor is passed into a large distillation column and fractionated, typically into four or five main streams. Each of these streams can be subjected to downstream treatment to separate and purify individual products. The overhead from the main column usually contains mostly C.sub.5 and lighter hydrocarbons, with other contaminants, such as hydrogen, carbon dioxide, water vapor, hydrogen sulfide, ammonia, nitrogen and other trace materials. Because of the conditions under which the cracking takes place, this fraction contains substantial amounts of unsaturated hydrocarbons, particularly ethylene and propylene. The overhead stream is separated into overhead liquids, which form gasoline product, and overhead vapor, generally known as wet gas. The wet gas passes to the unsaturated gas treatment section, where various absorbers and distillation columns are used to recover additional gasoline components and C.sub.3 /C.sub.4 hydrocarbons. The light gases remaining after treatment are sent to the fuel line.
Fuel gas from FCC units may represent as much as 3-4 wt % of the plant feedstock. A representative typical composition, by volume, is about 15-30% hydrogen, about 15-40% ethylene and propylene, about 5-20% C.sub.3+ hydrocarbons and about 30-60% C.sub.1 /C.sub.2 hydrocarbons. The demand for ethylene and propylene as chemical feedstocks is rising, as is the demand for hydrogen to supply the hydrogen-consuming units, such as hydrotreaters and crackers, in the refinery. In fact, most refineries currently operate with a hydrogen deficit, which would be reduced if more hydrogen could be recovered from ongoing operations. Thus, these fuel gas streams present the opportunity for additional recovery of needed and valuable materials, if cost-effective separation techniques can be found.
In addition, only a finite quantity of fuel gas is needed, so some plants are bottlenecked by over supply. In these bottleneck situations, reduction in the amount of fuel gas produced, and/or control of the Btu value of that gas by reducing the heavier hydrocarbon content, would enable throughput of the unit operations in the refinery train, such as hydrotreating or reforming, to be increased.
Hydrogen recovery techniques that have been deployed in refineries include, besides simple phase separation of fluids, pressure swing adsorption (PSA) and membrane separation. U.S. Pat. No. 4,362,613, to Monsanto, describes a process for treating the vapor phase from a high pressure separator in a hydrocracking plant by passing the vapor across a membrane that is selectively permeable to hydrogen. The process yields a hydrogen-enriched permeate that can be recompressed and recirculated to the hydrocracker reactor. U.S. Pat. No. 4,367,135, also to Monsanto, describes a process in which effluent from a low pressure separator is treated to recover hydrogen using the same type of hydrogen-selective membrane. U.S. Pat. No. 4,548,619, to UOP, shows membrane treatment of the overhead gas from an absorber treating effluent from benzene production. The membrane again permeates the hydrogen selectively and produces a hydrogen-enriched gas product that is withdrawn from the process. U.S. Pat. No. 5,053,067, to L'Air Liquide, discloses removal of part of the hydrogen from a refinery off-gas to change the dewpoint of the gas to facilitate downstream treatment. U.S. Pat. No. 5,157,200, to Institut Francais du Petrole, shows treatment of light ends containing hydrogen and light hydrocarbons, including using a hydrogen-selective membrane to separate hydrogen from other components. U.S. Pat. No. 5,689,032, to Krause/Pasadyn, discusses a method for separating hydrogen and hydrocarbons from refinery off-gases, including multiple low-temperature condensation steps and a membrane separation step for hydrogen removal. U.S. Pat. No. 5,332,492, to UOP, concerns treatment of effluent gases from catalytic reformers by cooling followed by PSA.
The use of certain polymeric membranes to treat off-gas streams in refineries is also described in the following papers: "Hydrogen Purification with Cellulose Acetate Membranes", by H. Yamashiro et al., presented at the Europe-Japan Congress on Membranes and Membrane Processes, June 1984; "Prism.TM. Separators Optimize Hydrocracker Hydrogen", by W. A. Bollinger et al., presented at the AIChE 1983 Summer National Meeting, August 1983; "Plant Uses Membrane Separation", by H. Yamashiro et al., in Hydrocarbon Processing, February 1985; and "Optimizing Hydrocracker Hydrogen" by W. A. Bollinger et al., in Chemical Engineering Progress, May 1984. These papers describe system designs uses cellulose acetate or similar membranes that permeate hydrogen and reject hydrocarbons. The use of membranes in refinery separations is also mentioned in "Hydrogen Technologies to Meet Refiners' Future Needs", by J. M. Abrardo et al. in Hydrocarbon Processing, February 1995. This paper points out the disadvantage of membranes, namely that they permeate the hydrogen, thereby delivering it at low pressure, and that they are susceptible to damage by hydrogen sulfide and heavy hydrocarbons.
A chapter in "Polymeric Gas Separation Membranes", D. R. Paul et al. (Eds.) entitled "Commercial and Practical Aspects of Gas Separation Membranes", by Jay Henis describes various hydrogen separations that can be performed with hydrogen-selective membranes.
In all of the above cases, the membranes used to perform the hydrogen/hydrocarbon separation are hydrogen-selective, that is, they permeate hydrogen preferentially over hydrocarbons and all other gases in the mix.
A difficulty that hampers the use of membrane separation systems of the type described above is the presence in off-gases of the heavier hydrocarbons, water vapor and hydrogen sulfide. These materials cause a variety of problems, including catastrophic collapse of the membranes. For example, a report by N. N. Li et al. to the Department of Energy ("Membrane Separation Processes in the Petrochemical Industry", Phase II Final Report, September 1987) presents data showing the effect of water vapor on membrane flux for cellulose acetate membranes, and concludes that "for relative humidities of 30% and higher, the flux decline is large, rapid, and irreversible". E. W. Funk et al. ("Effect of Impurities on Cellulose Acetate Membrane Performance", Recent Advances in Separation Techniques--III, AIChE Symposium Series, 250, Vol 82, 1986) advocate that "Moisture levels up to 20% RH appear tolerable but higher levels can cause irreversible membrane compaction". Similar or worse problems can occur if liquid hydrocarbons are allowed to come into contact with membranes surfaces, as well as glues or other components used in the membrane modules. Although the feed gas to the inlet of the membrane separation system may be comfortably above its dewpoint, as the gas travels along the modules and is depleted in the faster permeating hydrogen, the hydrocarbon content of the residue can quickly build up, raising the dewpoint temperature sufficiently for hydrocarbon condensation in the modules to take place. To avoid this, either the gas must be heated at least 10.degree. C. above the highest dewpoint temperature that will be reached, or contaminants must be removed to a low level before the gas enters the membrane system. Literature from Membrane Associates Ltd., of Reading, England, shows and describes a design for pooling and downstream treating various refinery off-gases, including passing of the membrane permeate stream to subsequent treatment for LPG recovery.
Other references that describe membrane-based separation of hydrogen from gas streams in a general way include U.S. Pat. Nos. 4,654,063, 4,836,833, to Air Products, and U.S. Pat. No. 4,892,564, to Cooley.
U.S. Pat. No. 5,332,424, to Air Products, describes fractionation of a gas stream containing light hydrocarbons and hydrogen using an "adsorbent membrane". The membrane is made of carbon, and selectively adsorbs hydrocarbons onto the carbon surface, allowing separation between various hydrocarbon fractions to be made. Hydrogen tends to be retained in the membrane residue stream. Other Air Products patents that show application of carbon adsorbent membranes to hydrogen/hydrocarbon separations include U.S. Pat. Nos. 5,354,547; 5,435,836; 5,447,559 and 5,507,856, which all relate to purification of streams from steam reformers.
U.S. Pat. No. 4,857,078, to Watler, mentions that, in natural gas liquids recovery, streams that are enriched in hydrogen can be produced as retentate by a rubbery membrane.
U.S. Pat. No. 4,772,295, to Nippon Kokan Kabushiki Kaisha, discloses a process using a membrane loop to recirculate hydrocarbon enriched vapors to an absorption column.
Treatment schemes for gas mixtures that include light olefin components and hydrogen are known. U.S. Pat. No. 5,634,354, to Air Products, discloses recovery of hydrogen and olefins from gas streams containing mixtures of the same. The recovery process uses a membrane separation system to remove hydrogen from the gas mixture, followed by low-temperature condensation of the hydrocarbons. The membrane used to perform the separation is either a polymeric membrane selective for hydrogen over hydrocarbons or a carbon adsorbent membrane selective for hydrocarbons over hydrogen. Separation of refinery waste gases by means of adsorbent membrane/PSA hybrid systems is described in some detail in reports by M. Anand and K. A. Ludwig to the U.S. Department of Energy ("Novel Selective Surface Flow Membranes for the Recovery of Hydrogen from Waste Gas Streams", Phase I (1995) and Phase II (1996) Final Reports under contract number DE-FC04-93AL94461). Separation of such gases, also containing olefins, is discussed in materials distributed at a U.S. Department of Energy, Office of Industrial Technology, exhibit in Washington, D.C. ("Scale-Up of Selective Surface Flow Membrane for Gas Separation", T. Nahieri et al., Air Products and Chemicals, 1996). U.S. Pat. No. 5,082,481, to Lummus Crest, describes removal of carbon dioxide, hydrogen and water vapor from a cracking effluent light gas stream, the hydrogen separation being accomplished by a hydrogen-selective membrane. U.S. Pat. No. 5,452,581, to Dinh et al., describes the use of a hydrogen-selective membrane as part of the ethylene recovery system for treating effluent from an ethylene manufacturing facility. The membrane is used to remove hydrogen to raise the hydrocarbon dewpoint of the gas, thereby facilitating olefin recovery.